Adsorption Of Volatile Organic Compounds Derived From Organic Matter

ABSTRACT

A method for adsorbing volatile organic compounds (VOCs) derived from organic matter comprises adsorbing the VOCs onto palladium doped ZSM-5, optionally at ambient temperature. The organic matter can be perishable organic goods such as food, including fruit and/or vegetables, horticultural produce, including plants and/or cut flowers, or refuse. The palladium doped ZSM-5 has a Si:Al ratio of less than or equal to 100:1 and preferably has a palladium content of from 0.1 wt % to 10.0 wt % based on the total weight of the doped ZSM-5.

This invention relates to the adsorption of volatile organic compounds(VOCs) derived from organic matter. More particularly, the organicmatter can be perishable organic goods, such as food.

VOCs are a wide ranging class of compounds including environmentalpollutants such as certain components of car exhaust gases, solvents andaerosol gases, but also including a range of compounds that are derivedfrom organic matter. One example of a VOC derived from organic matter isethylene, a plant hormone that causes ripening, whilst another exampleis trimethylamine, a gas commonly given off by fish as it decomposes.

The removal of VOCs derived from organic matter is of interest for avariety of applications. The adsorption of ethylene can preventundesired ripening and softening, loss of colour, loss of leaves andsprouting to occur in fruit and vegetables, it is also known to preventother food and horticultural products from perishing prematurely, andcan help eliminate unpleasant smells.

Various methods have been used to oxidise or combust VOCs using Pt onAl₂O₃ or KMnO₄. However, although these systems are efficient for theremoval of VOCs, they have disadvantages associated with their use. Pton Al₂O₃ works by catalytically combusting the ethylene at elevatedtemperatures, therefore Pt on Al₂O₃ needs to be used in a heated unitseparate from the source of the VOCs (see for example GB 2 163 637 A andU.S. Pat. No. 4,331,693). KMnO₄ cannot remove VOCs efficiently fromhumid environments (see Example 4). Since organic matter, such as food,cannot be heated without being altered and inherently exudes moisturesuch systems are unsuitable for use in removing VOCs derived fromorganic matter.

Other methods used to remove VOCs are suited for use at lowertemperatures; these include the use of high surface area supports,usually in conjunction with a promoter, for the adsorption of VOCs. ForExample, JP 2-261341 discloses the adsorption of ethylene fromrefrigerated storage compartments, JP 2-233381 discloses an ethyleneadsorption film and JP 2000-004783 discloses a combined ethyleneadsorber, deodoriser and anti-bacterial product for use in arefrigerator. Specific support materials are not disclosed in any ofthese publications, instead activated carbon and metal oxides are statedas being generally suitable for use as supports. GB 2 252 968 A relatesto an adsorber comprising a sepiolite in combination with a zeolite, andoptionally a metal selected from the platinum group metals, the irongroup metals, group I metals, group VII metals and the rare earthmetals. The most preferred zeolites for use in the invention describedin GB '968, are silicalites because their alumina content is almostzero.

We have now developed a catalyst system capable of removing VOCs derivedfrom organic matter at ambient temperatures, or temperatures at whichorganic goods such as food are chilled or refrigerated to prolong shelflife, by adsorbing said gases more efficiently than by those systemsdisclosed in the prior art.

In accordance with a first aspect of the present invention, there isprovided the use of palladium doped ZSM-5 to adsorb VOCs derived fromorganic matter, wherein the Si:Al ratio of the ZSM-5 is less than orequal to 100:1. Optionally the Si:Al ratio of the ZSM-5 is from 22:1 to28:1.

At least a proportion of the adsorbed VOCs may be converted intosecondary compounds after adsorption onto the doped ZSM-5.

In one embodiment the organic matter consists of perishable organicgoods, such as items of food and horticultural produce. The items offood may comprise fruit and/or vegetables. The horticultural produce maycomprise plants and/or cut flowers.

In another embodiment the organic matter comprises refuse. Such refusemay include kitchen refuse such as waste food, which produces unpleasantodours whilst decomposing.

The organic matter from which the VOCs are derived may be containedwithin a storage container or package, such that the doped ZSM-5 has aclosed or semi-enclosed environment within which to adsorb the VOCs. Inthe case of perishable organic goods the storage container or package islikely to be the container or package within which the goods arecontained, e.g. crates used to store the goods when in transit or thepackaging within which the goods are kept when on display prior topurchase. In another embodiment, the doped ZSM-5 is incorporated into,or into part of, the storage container or package itself. In a furtherembodiment, the doped ZSM-5 is incorporated into a label comprising asubstrate for insertion and retention within a storage container orpackage.

If the perishable organic goods comprise items of food, the doped ZSM-5may be packaged in a way to prevent direct contact with the food, e.g.behind a gas permeable barrier layer. The gas permeable barrier layermight form part of a sachet or label enclosing powdered doped ZSM-5 orthe gas permeable layer could be affixed on top of a layer of inkcomprising doped ZSM-5. The ink could be fixed to an internal surface ofthe storage container or package by printing, casting, rollerapplication, brushing, spraying or like techniques. Additionally as theadsorption capacity of doped ZSM-5 is moderately sensitive to thepresence of water (see Example 4), the doped ZSM-5 may be packaged witha water adsorbing material, such as silica gel.

If, however, the source of VOCs is refuse, the storage container orpackage may be a refuse receptacle.

Commonly the doped ZSM-5 will be particulate and may be looselypackaged, such as within a sachet (see above). Alternatively, theparticulate may be associated with another object, such as by beingincorporated into a storage container, incorporated into an ink (seeabove) or simply coated onto another object, e.g. a ceramic or metalmonolith, such as those used as catalyst carriers. Other forms of lowpressure-drop substrates, such as those commonly used as catalystcarriers, may also be used. In another embodiment the doped ZSM-5 is inthe form of extrudates, pellets, tablets, grains or granules. The ZSM-5may be doped before or after being formed into such extrudates, pellets,tablets, grains or granules.

Other methods of using the present invention may be used in appropriatecircumstances.

One advantage associated with this invention is that the VOCs can beadsorbed at relatively low temperatures, such as in the range of from−10° C. to 50° C., more commonly from 0° C. to 30° C. This enables thedoped ZSM-5 to be used in the environment within which the organicmatter is commonly found, e.g. refrigerators or at ambient temperature,without requiring complex heating and air recirculation equipment to beused. Nonetheless, where a particular application allows for heating andair recirculation equipment to be used (e.g. an air conditioning system)the doped ZSM-5 may also be operated at an elevated temperature, e.g.above 60° C.

In one embodiment the VOCs comprise ethylene. Ethylene is a gaseoushormone released by plants that can cause plants to wilt and fruits toripen. The removal of VOCs produced by plants can delay these processesenabling food and horticultural produce to be kept in transit and/or instorage for longer without accelerating perishing. Therefore, aparticular application of this invention is to industries that produce,ship, export and buy food and horticultural produce. Initial tests havesuggested that, unlike prior art methods, the use of an adsorberaccording to this invention could enable the shelf life ofpost-climacteric fruit to be extended (Terry L, Ilkenhans T, Poulston S,Rowsell E and Smith A W J, Postharvest Biol. and Tech.—submitted). Thatis, even after the climacteric respiratory rise has been initiated,fruit may be prevented from ripening further (or at least the rate ofripening slowed) using palladium doped ZSM-5 to adsorb ethylene.

In another embodiment the VOCs comprise formaldehyde and/or acetic acid.Formaldehyde and acetic acid are malodorous chemicals that are oftenfound in the home. Formaldehyde may be released from pressed bonded woodproducts, such as plywood, but is also found in dyes, textiles,plastics, paper products, fertilizer, and cosmetics. Acetic acid may bereleased from kitchen waste and animal waste. Therefore, one potentialapplication of this invention is to the removal of malodours from thedomestic environment.

Another point of interest is that, although there is some loss ofactivity in the palladium doped ZSM-5 once they have been exposed towater, they are still able to function efficiently when “wet”. As foodand horticultural produce are usually stored in humid environments, thisfeature is also advantageous to the relevant industries.

Methods of manufacturing palladium doped ZSM-5 are known to the skilledchemist, and include the use of a variety of palladium salts, such asPd(NO₃)₂, Pd(OCH₃CO₂)₂ and PdCl₂. Commonly the ZSM-5 will be calcinedafter impregnation with at least one palladium salt, however, for someapplications this may not be necessary. Samples of palladium doped ZSM-5that are calcined will comprise at least partially oxidised palladium.

The palladium itself can comprise from 0.1 wt % to 10.0 wt % based onthe total weight of the ZSM-5, optionally from 0.5 wt % to 5.0 wt %based on the total weight of the ZSM-5.

In one embodiment, the doped ZSM-5 is effective to adsorb the VOCs to alevel of less than or equal to 0.10 ppm, optionally to a level of lessthan or equal to 0.05 ppm.

Another advantage of this invention is that the doped ZSM-5 may be usedcontinuously for VOC removal for an extended period of time, e.g.several days, (the actual time depending upon the environment withinwhich it is used). Furthermore, after use the ZSM-5 may be heated to250° C. for 30 minutes in air to release the VOCs adsorbed on the ZSM-5and any secondary compounds present, thus regenerating the palladiumdoped ZSM-5 for further use. This enables the palladium doped ZSM-5 tobe used for extended periods of time, then removed from the source ofVOCs, regenerated and re-used. As the regeneration process is neitherlengthy nor costly, this means the doped ZSM-5 is a cost effectiveproduct for VOC removal. It is worth noting that, by contrast,regeneration of KMnO₄ is not possible as the material decomposes onheating to K₂O and manganese oxide(s).

In order to identify the time when the doped ZSM-5 has reached its VOCadsorption capacity and therefore needs regenerating, a VOC indicatormay be included for use with the doped ZSM-5. Suitable indicatorsinclude the palladium based ethylene indicator disclosed in patentapplication JP 60-202252.

In accordance with a second aspect of the present invention, there isprovided palladium doped ZSM-5, wherein the Si:Al ratio of the ZSM-5 isless than or equal to 100:1 and the palladium comprises from 0.1 wt % to10.0 wt % based on the total weight of the doped ZSM-5. Optionally theSi:Al ratio of the ZSM-5 is from 22:1 to 28:1 and/or the palladiumcomprises from 0.5 wt % to 5.0 wt % based on the total weight of thedoped ZSM-5.

In order that the invention may be more fully understood the followingnon-limiting Examples are provided by way of illustration only and withreference to the accompanying drawings in which:

FIG. 1 is a graph showing ethylene adsorption over time by ZSM-5 dopedwith palladium (with and without water present in the gas feed, wet ordry) and un-doped ZSM-5, said graph demonstrating that it is thepresence of palladium doping that enables ethylene adsorption;

FIG. 2 is a graph showing ethylene adsorption by ZSM-5 with a SiO₂:Al₂O₃ratio of 23 with different levels of palladium doping, varying from 0.5wt % to 5 wt %, and for comparison silver doping at 2.5 wt %, said graphdemonstrating the effectiveness of palladium doping over that of anothermetal and the variation in ethylene adsorption capacity with a change inlevel of doping;

FIG. 3 is a graph showing ethylene adsorption by different palladiumdoped zeolites (SiO₂:Al₂O₃ ratios given in brackets), the Pd loading inall cases if 2.5 wt %, said graph showing that ethylene adsorption bypalladium doped zeolites is greatest for ZSM-5 samples with a relativelylow SiO₂:Al₂O₃ ratio;

FIG. 4 is a graph showing the CO₂ and ethylene concentrations measuredin an example using a banana as the organic matter from which theethylene is derived (see Example 5 for further discussion); and

FIG. 5 is a graph showing ethylene adsorption by a monolith coated with2.5 wt % Pd/ZSM-5; and

FIG. 6 is a graph showing the lightness of an ethylene indicator afterexposure to an apple, an apple and adsorber, and the indicator on itsown (with a reference measurement of the indicator on its own afterexposure to ethylene).

EXAMPLE 1 Preparation of Doped Supports

Doped supports, also known as adsorbers, were prepared using theincipient wetness impregnation method. Typically 20 g of the support(e.g. the hydrogen form of the zeolite) was impregnated with the nitratesalt or chloride salt of the appropriate metal (e.g. palladium), andthen dried at 110° C. before being calcined in air at 500° C. for 2 hrs.

EXAMPLE 2 Ethylene Adsorption Measurements

Measurements were carried out in a plug flow reactor at 21° C. with 0.1g doped support of particle size 250-355 μm with a flow rate of 50ml/min of gas comprising 10% O₂, 200 ppm C₂H₄, ˜1% water (where present)and balance He/Ar.

EXAMPLE 3 Ethylene Adsorption by Pd Doped onto a Variety of Supports

Samples 4.0 wt % Pd doped activated carbon and 2.5 wt % Pd/ZSM-5(23)were made according to Example 1 (using palladium chloride salt andpalladium nitrate respectively) and various activated carbons. Thesamples were tested for their ethylene adsorption capacity, inaccordance with Example 2. The results are set out below:

Ethylene Adsorber adsorption/μl g⁻¹ Pd/ZSM-5 32228 PdCl/C (black pearl)372 PdCl/C (denka) 80 PdCl/C (vulcite) 132 PdCl/C (ketjen) 292 PdCl/C(xc-72R) 205

This experiment shows that Pd/ZSM-5 has a far higher adsorption capacitythan Pd doped activated carbon.

EXAMPLE 4 “Wet” Ethylene Adsorption by Metal Doped ZSM-5 and KMnO₄ onAl₂O₃

Samples of 2.5 wt % Pd/ZSM-5(23), made according to Example 1, andsamples of 5 wt % KMnO₄ on Al₂O₃ (Condea, 140 m²/g) were tested fortheir ethylene adsorption capacity, in accordance with Example 2. Thematerials were tested when dry and after having been exposed to water bybeing placed in a dessicator containing water at ambient temperature fora set period of time. The results of this experiment are set out in thetable below:

Ethylene Adsorber Pre-treatment adsorption/μl g⁻¹ Pd/ZSM-5 Calcined inair at 500° C. 4162 Pd/ZSM-5 Calcined in air at 500° C., exposed 3753 towater vapour for 100 hrs at 21° C. KMnO₄/Al₂O₃ Dried 110° C. 750KMnO₄/Al₂O₃ Dried 110° C., exposed to water 0 vapour for 72 hrs at 21°C.

Additionally, samples of 2.5 wt % M/ZSM-5, M=Pt, Co, Ni, Rh, Ru, Ir, Mo,Cu, W, V, and Au, (all with a SiO₂:Al₂O₃ ratio of 23) were madeaccording to Example 1 and tested for their ethylene adsorption capacityafter having been exposed to water as above. The ethylene adsorptioncapacities measured were less than 60 μl g⁻¹ catalyst for all of thesamples.

This experiment shows that the palladium doped zeolite only losesapproximately 10% of its dry ethylene adsorption capacity when wet. Allthe other metals tested show negligible ethylene adsorption when wet,whilst KMnO₄ on Al₂O₃ loses all of its ethylene adsorption function whenwet.

EXAMPLE 5 Adsorption of Ethylene from Fruit

A banana (weighing approximately 150 g) was placed in an airtight vesselof volume 1.15 litres and left for approximately 1 day. Increase in CO₂and ethylene concentration was measured as a function of time using GasChromatography. The experiment was then repeated with 0.2 g adsorber(2.5 wt % Pd/ZSM-5) present in the vessel.

As can be seen with reference to FIG. 4, the banana alone led to anapproximately linear increase in both CO₂ and ethylene concentrations,whereas when the adsorber was present there was no detectable increasein ethylene concentration whilst the concentration of CO₂ increased atapproximately the same rate as before indicating a similar respirationrate.

Further experiments were carried out with a variety of fruit beingplaced in the same airtight vessel and left for approximately 20 hoursto yield the following results:

Ethylene Fruit Fruit weight/g Adsorber Concentration/ppm Banana 140 none5.5 Banana 140 un-doped ZSM-5 (23) 3.9 Banana 156 1 wt % Pd/ZSM-5 (23)0.0 Banana 137 2.5 wt % Pd/ZSM-5 (23) 0.0 Peach 114 none 35.0 Peach 1142.5 wt % Pd/ZSM-5 (23) 1.5 Apple 148 none 316.4 Apple 148 1 wt %Pd/ZSM-5 (23) 17.2 Tomato 208 none 1.4 Tomato 207 2.5 wt % Pd/ZSM-5 (23)0.0 Pear 156 none 42.9 Pear 156 1 wt % Pd/ZSM-5 (23) 1.7 Passion 60.9none 109.9 Fruit Passion 60.6 2.5 wt % Pd/ZSM5 (23) 13.7 Fruit

EXAMPLE 6 Ethylene Adsorption Using a Monolith

A 900 cpsi (cells per square inch) cordierite catalyst monolith, of thetype commonly used in vehicle exhaust catalysts, weighing 3 g withdimensions of 2.2 cm diameter and 2.5 cm length, was coated with a 2.5wt % Pd/ZSM-5 slurry. The slurry was prepared using finely milled dopedZSM-5 suspended in water (the doped ZSM-5 was prepared according to themethod described in Example 1). The washcoat load was 0.28 g/cm². Themonolith was tested for its ethylene adsorption capacity in an ITK rigat a flow rate of 10 ml/min using gas comprising 10% O₂, 20 ppm C₂H₄ andbalance Ar. The results of the test may be seen in FIG. 5

This experiment shows that the adsorber coated monolith is able toremove almost all the ethylene present over the course of several days.(Additional experiments showed that the ethylene adsorption rate speededup when the temperature at which the experiment was carried out wasincreased).

EXAMPLE 7 Ethylene Adsorption in the Presence of an Indicator

An ethylene indicator was prepared following patent application JP60-202252 (essentially an acidified solution of ammonium molybdate andpalladium sulphate impregnated onto a porous support). When exposed toethylene this material changed colour from light yellow to darkblue/black.

0.5 g of indicator was placed in a 1 litre glass beaker on its own, withonly an apple, and with an apple and 0.2 g of ethylene adsorber beingpresent (i.e. beaker 1=sensor only, beaker 2=fruit+indicator and beaker3=fruit+adsorber+indicator). Each beaker was sealed with cling film andleft for 72 hours. At 24 hour intervals, each ethylene sensor powder wasremoved and the colour measured on a Spectroflash 500 seriescalorimeter. The CIELAB Lightness scale (L) was used to monitor thechange in lightness of the sensor powder, where a value of 100 is whiteand a value of 0 is black.

A sample of the ethylene indicator was also exposed to 1000 ppm ethylenefor 24 hours. The colour measurements of this sample and a fresh samplewere also recorded for reference.

As can be seen with reference to FIG. 6, ethylene from the apple withoutscavenger has darkened the indicator after 72 hours to almost the sameextent as the sample of the ethylene indicator exposed to 1000 ppmethylene for 24 hours. The colour of the sensor powders in beakerscontaining the fruit with the adsorber have not darkened as much,showing that the ethylene adsorber is removing ethylene. Samples ofethylene sensor sealed in empty beakers did not change coloursignificantly over the 72 hours.

EXAMPLE 8 Formaldehyde and Acetic Acid Adsorption

Measurements were carried out using a saturator at 21° C. with 0.1 gdoped ZSM-5(23) of particle size 250-355 μm with a flow rate of 50ml/min of gas comprising 10% O₂, 300 ppm CH₂O or CH₃COOH and balanceHe/Ar.

The formaldehyde adsorption capacity of 2.5 wt % Pd/ZSM-5(23) was foundto be 9750 μl/g adsorber. The acetic acid adsorption capacity of 2.5 wt% Pd/ZSM-5(23) was found to be 29241 μl/g adsorber.

1. A method for adsorbing volatile organic compounds (VOCs) derived fromorganic matter comprising the step of adsorbing the VOCs onto palladiumdoped ZSM-5, wherein the Si:Al ratio of the ZSM-5 is less than or equalto 100:1.
 2. The method according to claim 1, wherein the Si:Al ratio ofthe ZSM-5 is from 22:1 to 28:1.
 3. The method according to claim 1,wherein the organic matter consists of perishable organic goods.
 4. Themethod according to claim 3, wherein the perishable organic goodscomprise items of food.
 5. The method according to claim 4, wherein theitems of food comprise at least one of fruit and vegetables.
 6. Themethod according to claim 3, wherein the perishable organic goodscomprise horticultural produce.
 7. The method according to claim 6,wherein the horticultural produce comprises at least one of plants andcut flowers.
 8. The method according to claim 1, wherein the organicmatter comprises refuse.
 9. The method according to claim 1, wherein theorganic matter is contained in a storage container or package.
 10. Themethod according to claim 9, wherein the palladium doped ZSM-5 isincorporated into, or into part of, the storage container or package.11. The method according to claim 9, wherein the palladium doped ZSM-5is incorporated into a label comprising a substrate for insertion andretention within the storage container or package.
 12. The methodaccording to claim 9, wherein the storage container or package is arefuse receptacle.
 13. The method according to claim 1, wherein the VOCsare adsorbed at a temperature of from −10° C. to 50° C.
 14. The methodaccording to claim 13, wherein the VOCs are adsorbed at a temperature offrom 0° C. to 30° C.
 15. The method according to claim 1, wherein theVOCs comprise ethylene.
 16. The method according to claim 1, wherein theVOCs comprise at least one of formaldehyde and acetic acid.
 17. Themethod according to claim 1, wherein the palladium comprises from 0.1 wt% to 10.0 wt % based on the total weight of the doped ZSM-5.
 18. Themethod according to claim 17, wherein, the palladium comprises from 0.5wt % to 5.0 wt % based on the total weight of the doped ZSM-5.
 19. Themethod according to claim 1, wherein the VOCs are adsorbed to a level ofless than or equal to 0.10 ppm.
 20. The method according to claim 19,wherein the VOCs are adsorbed to a level of less than or equal to 0.05ppm.
 21. The method according to claim 1, wherein the ZSM-5 is heated to250° C. for 30 minutes in air to release the VOCs adsorbed on the ZSM-5and any secondary compounds present, thus regenerating the doped ZSM-5for further use.
 22. The method according to claim 1, wherein the dopedZSM-5 is used with a VOC indicator.
 23. Palladium doped ZSM-5, whereinthe Si:Al ratio of the ZSM-5 is less than or equal to 100:1 and thepalladium comprises from 0.1 wt % to 10.0 wt % based on the total weightof the doped ZSM-5.
 24. Palladium doped ZSM-5 according to claim 23,wherein the Si:Al ratio of the ZSM-5 is from 22:1 to 28:1.
 25. Palladiumdoped ZSM-5 according to claim 23, wherein the palladium comprises from0.5 wt % to 5.0 wt % based on the total weight of the doped ZSM-5. 26.Palladium doped ZSM-5 according to claim 23, wherein the palladium dopedZSM-5 is in the form of a label, sachet or ink, or coated on a catalystcarrier, or in the form of extrudates, pellets, tablets, grains orgranules.